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CHM 5423 – Atmospheric Chemistry
Notes on reactions of organics in the troposphere (Chapter 5)
5.1 Introduction
In general, the lifetime of a molecule in the troposphere is governed by a variet of processes. Some
molecules decompose by photodissociation. Other molecules, particularly polar molecules, can be physically
removed from the troposphere by rainout, washout, and deposition. Molecules can also be removed by chemical
reaction. Finally, molecules with no significant removal processes in the troposphere can migrate into the
stratosphere. In this chapter, we focus on the chemical reactions of organic molecules.
Organic molecules are an important trace constituent in the lower atmosphere of the Earth. Organic
molecules have both natural and anthropogenic sources, and can be released directly into the atmosphere as a
primary pollutant, or can form from the trnsformation of other organic molecules as secondary pollutants. In this
chapter we examine the main reactions of general classes of organic molecules in the troposphere.
5.2 Oxidizing species in the troposphere
While the general tendency of organic molecules in the troposphere is to go from reduced to oxidized
molecules, the reactions do not begin with the direct reaction of molecules with molecular oxygen. Instead, reactions
are initiated with oxidant molecules, generally radical species that exist at low concentration. In this section we
indicate the major oxidant species found in the troposphere.
OH (daytime)
Hydroxyl radical is the main oxidizing species in the troposphere. It exists at extremely low concentrations,
typically ~ 1 x 106 molecules/cm3, with large fluctions around this value. Hydroxyl radical also has an extremely
short lifetime in the troposphere. Because of this, and because the major sources of hydroxyl radicals are all
photochemical reactions, it is an important reactant in the daytime, but essentially absent at night.
The main source of hydroxyl radical in the troposphere is from photodissociation of ozone, followed by
reaction of excited oxygen atoms with water molecules.
O3 + h ( < 340. nm)  O(1D) + O2(1) or O2(3)
(2.1)
O(1D) + H2O  2 OH
(2.2)
The threshold wavelength for photodissociation of ozone to produce O( 1D) + O2(1) is 310. nm. At wavelengths
shorter than this, the quantum yield for production of O( 1D) is approximately 0.9. The other main photodissociation
pathway is
O3 + h  O(3P) + O2(3)
(2.3)
which produces ground state products. The oxygen atom produced by reaction 2.3 does not have sufficient energy to
react with water molecules, and usually reforms ozone by the termolecular recombination reaction
O(3P) + O2(3) + M  O3 + M
(2.4)
At wavelengths longer than 310. nm the quantum yield for the production of O(1D) atoms was expected to quickly
drop to zero. In fact, while the quantum yield decreases, it remains in the region 0.1-0.2 (Fig 3.3, Chapter 3). This is
because a second, spin forbidden process that forms an O( 1D) atom and a O2(3) molecule occurs.
The fate of the electronically excited oxygen atom produced in reactin 2.1 is either quenching, to form a
ground state oxygen atom, or reaction with a water molecule to form two hydroxyl radicals. The fraction of O( 1D)
atoms forming hydroxyl radical is a sensistive function of relative humidity, which governs the concentration of
water molecules in the gas phase.
Two other minor sources of hydroxyl radical are also photochemical processes. They are
1
HONO + h ( < 400. nm)  OH + NO
(2.5)
H2O2 + h ( < 370. nm)  2 OH
(2.6)
Photodissociation of organic peroxides (ROOH) can also produce hydroxyl radical, but concentrations of organic
peroxides are generally lower than the concentration of hydrogen peroxide.
It is estimated that ~ 80% of OH radical production is from ozone photodissociation, with the remaining
molecules generated by reactions 2.5, 2.6, and other minor processes. While there are a few non-photochemical
sources of hydroxyl radical, they are much less important, though they do generate small concentrations of hydroxyl
radical at night.
O3 (daytime + nighttime)
The main source of ozone in the troposphere is photodissiciation of nitrogen dioxide
NO2 + h ( < 420. nm)  NO + O(3P)
(2.7)
followed by the termolecular recombination reaction forming ozone. Nitrogen oxides are often found as a pollutant
in urban areas, and so ozone concentrations in such regions are generally higher than in rural areas. In fact,
concentrations of NO, NO2 and O3 in the daytime are linked by the Leighton mechanism, which will be discussed
later.
While ozone is produced by a photochemical process, it has a half-life in the troposphere of a few days, and
so is present both in the daytime and at night. A typical ozone concentration is ~ 1. x 1012 molecule/cm3, with
fluctuations around this value between daytime and night time, and based on geographic location.
NO3 (night time)
Nitrogen trioxide is produced by the reaction of nitrogen dioxide with ozone
NO2 + O3  NO3 + O2
(2.8)
During the daytime nitrogen trioxide disappears due to photodissociation, which is fast due to the fact that the
molecule absorbs light throughout the visible region of the spectrum
NO3 + h ( <680. nm)  NO2 + O
(major pathway)
(2.9a)
 NO + O2
(minor pathway)
(2.9b)
At night, when photodissociation does not occur, concentrations of nitrogen trioxide gradually increase as NO 2 is
concerted to NO3 by reaction 2.8.
A typical concentration of NO3 is ~ 1. x 109 molecule/cm3.
HO2 (daytime)
Hydroperoxyl radical is generated photocheimcally in the troposphere. The main source of hydroperoxyl
radical is photodissociation of aldehydes. For example
HCHO + h ( < 370. nm)  H + CHO
 CO + H2
(major)
(2.10a)
(minor)
(2.10b)
HO2 is then generated by termolecular or bimolecular reaction of the major products of the photodissociation
reaction
2
H + O2 + M  HO2 + M
(2.11)
CHO + O2  HO2 + CO
(2.12)
Larger aldehydes are also capable of generating H and CHO radicals and so are additional sources of hydroperoxyl
radical.
Since the lifetime of hydroperoxyl radical is short, it is found at high concentrations only during the
daytime. Typical concentrations of hydroperoxyr radical are ~ 2.0 x 108 molecule/cm3.
Cl (daytime)
There is some evidence that Cl atoms can play a role in the chemistry of remote regions over the ocean.
Chlorine atoms can be generated by a number of processes, the most important of which is believed to be reaction
with nitrogen oxides such as dinitrogen pentoxide
N2O5 + NaCl(s)  NO2Cl + NaNO3(s)
(2.13)
NO2Cl + h ( < 480. nm)  NO2 + Cl
(2.14)
Chlorine atom concentrations are estimated to be ~ 1. x 10 2 molecules/cm3. Because chlorine atoms are expected to
be chemically reactive, they could play a role in the chemical processes over mid-ocean, where concentrations of
other oxidizing species are also low.
A summary of typical 24-hour average concentrations of major oxidizing species in the troposphere is given
in Table 2.1 It is important to remember that actual concentrations of these species vary widely from these "average"
values, and that in some cases average values themselves have a great deal of uncertainty. However, these average
values are useful in estimating half-lives for molecules in the troposphere.
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Table 2.1 - Typical concentrations of oxidizing species in the troposphere
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Molecule
Time present
24-hour average concentration (molecule/cm3)
OH
day
1. x 106
O3
day and night
1. x 1012
NO3
night
1. x 109
HO2
day
2. x 108
Cl
day
1. x 102
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